If you take a step, it's no longer standing tumbling. Go talk to my 9 year old Level 10 gymnasts, they do standing fulls with out a step-on balance beam.
But in cheerleading you only have running tumbling and standing tumbling. So step or not, if you're not running forward, it's still standing tumbling
Gymnasts always throw off the jump & tumbling sequence w/ those weird double arm swings (w/ out a step)..drives me crazy!